Reproducibility of the results is an essential requirement in science, and even more so when delicate issues such as impacts on Earth are discussed. We believe every reader should be given the opportunity to repeat our computations.
This is possible by using the free software OrbFit, developed by a consortium including the first author and his coworkers at the University of Pisa, M. Carpino (Obs. Milan/Brera), Z. Knezevic (Obs. Belgrade), and the second author. Other applications of OrbFit have been described in Paper I and [Milani et al. 1998]. This software is available at the anonymous ftp server copernico.dm.unipi.it, directory /pub/orbfit, it is distributed under the Free Software Foundation Public licence. In the latest versions of OrbFit, the authors of this paper have added to the interactive main FITOBS the capability to perform close approach analysis, such as those performed in this paper. In particular, the Figures of this paper have been generated from OrbFit output.
Acknowledgements We thank E. Bowell (Lowell Observatory) for supplying the simulated observations of the hypothetical killer asteroid 1992 KP4. The astrometric observations of the real asteroids used in the examples of this paper have been obtained from the Minor Planet Center, by using the Extended Computer Service. We thank the referees (J. Scotti and an anonymous one) for numerous suggestions to improve the readability.